Cathode



Feb. 3, 1959 D. R. KERSTETTER CATHODE Filed Nov. 16. 1953 INVENTOR DONALD R. KERSTETTER United States Patent 'CATHODE Donald R. Kerstetter, Emporium, Pa., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts This invention relates to a cathode and to an improved method of making the same. More particularly, it relates to an improved cathode structure for use in electron discharge devices and to the method of its preparation.

One of the problems confronting the design engineers in the electron discharge tube art has been that of finding a base material which has good electron emission qualities but one which does not have a high rate of sublimation. Since one normally finds that the materials which have the best electron emission are the ones which also normally have the highest rate of sublimation this has been a very difficult and somewhat unsurmountable problem up until this time. Another problem which has given considerable concern to the same design engineers has been the problem of leakage between the heater and the cathode. This leakage is, of course, traceable in part to the same sublimation problem. The higher the rate of sublimation from the base cathode material the greater the likelihood of heater to cathode leakage.

It is, accordingly, an object of this invention to provide a cathode structure which will permit the use of 2,872,611 Patented Feb. 3, 195

tion which has very good electron emission qualities.

Any of the well known nickel alloys may be used for Examples of such metals are aluminum, chromium,

materials which have good electron emission qualities while reducing the normal sublimation from the cathode sleeve and also indirectly reduce the heater cathode leakage.

A further object of this invention is to provide a method of treating a cathode structure in order to reduce sublimation from the cathode sleeve.

A still further object of this invention is to provide a cathode sleeve which will permit lower heater operating temperatures and consequently better heater life due to better heat transfer between the heater and the oathode.

In accordance with this invention it has been found that these objects and other advantages incidental thereto can be obtained by coating the normally uncoated end portions of a cathode sleeve with a metal whose oxide is of a refractory nature.

In the drawings which illustrate features of this invention Figure 1 is an enlarged front elevation showing a metal sleeve cathode which has been prepared in accordance with this invention.

Figure 2 is a perspective view of a lock seam cathode which has also been treated in accordance with this invention.

Figure 3 is a front elevation partly in section of an electron discharge device having a sleeve type cathode made in accordance with the invention.

The cathode of this invention is to be made in much the same manner as the cathode of the prior art, the difference being merely in a preliminary processing step and in the fact that a base material can be used for the cathode which has a very high emission characteristic even though it itself possesses a very high rate of sublimation. The base sleeve material may, for example, be a piece of seamless tubing made of nickel of a compositungsten, zirconium, magnesium and tantalum. These metals may be applied to the ends of the sleeve by electroplating or by vaporization technique. However, any method which will provide a very thin uniform adherent controlled coating is satisfactory for this purpose. The coating should preferably be on the order of .0002 mm. in thickness. Preferably the metal which is deposited on the base metal of the sleeve is diffused into the base metal in solution with the base metal. Subsequently, the metal oxide coating is formed on the surface when the coated sleeve is treated in a reducing atmosphere containing moderate amounts of water vapor. With the use of some other processing techniques, diffusion, alloying or oxidation of the type yielding satisfactory results may take place where only inert gas or vacuum conditions prevail. The metal such as shown at 14 in the drawing has been treated as indicated above to form the refractory oxide. This makes the surface of the end portion of the cathode sleeve which is covered by the refractory oxide comparatively non-conductive and thus increases the resistance to flow of current between the heater and the cathode. The oxide coating also acts as a barrier to the sublimation of the nickel of the base metal. As has already been brought out in the previous portions of the specification the advantages of applying the metal over the uncoated part of the cathode as against using a complex alloy in the basic sleeve material is that good electron emission qualities of the nickel sleeve material can be utilized even though the base material possesses a high rate of sublimation.

Other advantages are also obtained in the use of these metals. For example, when aluminum is used for this purpose in the treatment of lock seam cathodes the high temperature dielectric properties of this oxide can be used to advantage. In the preparation of the nickel strip from which the lock seam cathodes is formed it is also very difficult to prevent the aluminum from sublimingon both sides of the cathode strip. However, this is not a disadvantage for by allowing the aluminum to deposit on both sides of the strip the inside of the lock seam cathode is also covered to some extent. Since the aluminum oxide which is later formed is darker incolor that the original nickel a better heat transfer is possible. This permits lower heater operating temperature and consequently better heater life.

When aluminum is used as the coating metal the aluminum (1) forms a low vapor pressure surface thus reducing surface evaporation and (2) also forms an almost non-porous film over the base metal which lowers the rate of diffusion of readily volatile metals and thus in two ways decreases the rate of sublimation from the base metal.

Heater-cathode leakage would be reduced (1) because the inside of the cathode would have been covered with a layer of aluminum oxide which is a very poor primary emitter and (2) aluminum oxide is not only an insulating film but also keeps the heater coating from scraping on metal during insertion. The base sleeve material is then coated in the usual manner with a good emissive material such as barium carbonate or the triple carbonate mixture containing barium, strontium and calcium carbonates. This carbonate coating as shown at 12 and 24 is normally brought up to within a few centimeters of the ends of the seamless tubing, the

3 end being left bare and exposing the previously coated surface as shown at 14 and 22.

A lock seam cathode which has been prepared in accordance with this invention is shown in the enlarged view in Figure 2 of the -drawings. Thejbase' :nicke'l alloy material is coated 'as shown at 22'wi th a coating which appears on both ends and on the inside-of the cathode material.

Figure 3 shows an electron discharge device'havinga' glass envelope 30, contact prongs 32, mica spacers 34 and 36, a finished cathode with its coating 38 spaced out of contact with the mica 34 and 36 and its base metal with its refractory oxide coating 40 in contact with the mica 34 and 36. This .figure also shows the normal grid structure 4 and the anode 44 in relative position with one another.

As has been indicated previously in the specification the cathode of'this' invention can be made by coating the ends of the base metals alloy with a refractory metal. This can be done in many waysincluding v'aporization and electroplating techniques. When using these processes, however, it is advisable to provide some means for shielding that part of the base metal which is later to be covered with the emissive coating. When vaporization is the method employed, a simple shielding technique is sufiicient to do a-satisfact'ory job. When electroplating is resorted to the usual stopofi lacquer can be applied. In those cases in which aluminum has been used as the coating metal it has been found that a heat treatment at a temperature of 1500 F. for two minutes in a moist reducing atmosphere is suflicient to give good results.

While the above description and drawings submitted herewith disclose a preferred and practical embodiment of the cathode of this invention it will be understood that the specific details of construction and arrangement of parts as shown and described are by way of illustration and are not to be construed as limiting the scope of the invention.

What is claimed is:

A cathode for use in an electron discharge device comprising a tubular cathode sleeve of nickel, coatings of aluminum oxide on end-adjacent circumferentiallyextending bands of the outer periphery of said sleeve, and a coating of electron-emissive material on the outer periphery of said sleeve intermediate said bands.

References Cited in the file of this patent UNITED STATES PATENTS 1,671,953 .Gilson .May 29, 1928 1,880,092 Hull Sept. 27, 1932 1,890,911 Miessner Dec. 13, 1932 1,981,668 Ronci Nov. 20, 1934 2,014,787 Smithells et al. Sept. 17, 1935 2,053,090 Langvet al. Sept. 1, 1936 2,081,415 Veazie May 25, 1937 2,094,657 Kapteyn Oct. 5, 1937 2,123,686 Spencer July 12, 1938 2,527,984 Bruining et a1 Oct. 31, 1950 FOREIGN PATENTS 1,038,245 France May 6, 1953 

1. CATHODE FOR USE IN AN ELECTRON DISCHARGE DEVICE COMPRISING A RUBULAR CATHODE SLEEVE OF NICKEL, COATINGS OF ALUMINUM OXIDE ON END-ADJACENT CIRCUMFERENTIALLYEXTENDING BANDS OF THE OUTER PERIPHERY OF SAID SLEEVE 